In the quest for hydrocarbon reservoirs, companies employ many data-gathering techniques. The most detailed, albeit localized, data comes from well logging. During the well-drilling process, or shortly thereafter, driller pass logging instruments through the well bore to collect information about the surrounding formations. The information is traditionally collected in “log” form, i.e., a table, chart or graph of measured data values as a function of instrument position. The most sought-after information relates to the location and accessibility of hydrocarbon gases and fluids.
Resistivity, density, and porosity logs have proven to be particularly useful for determining the location of hydrocarbon gases and fluids. These logs are “open hole” logs, i.e., log measurements that are taken before the formation face is sealed with tubular steel casing. Acoustic logging tools provide measurements of acoustic wave propagation speeds through the formation. There are multiple wave propagation modes that can be measured, including compressional and flexural. Taken together, the propagation speeds of these various modes often indicate formation density and porosity.
Acoustic logging measurements are also valuable for determining the velocity structure of subsurface formations, which information is useful for migrating seismic survey data to obtain accurate images of the subsurface formation structure. Subsurface formations are often anisotropic, meaning that the acoustic waves propagation speed depends on the direction in which the wave propagates. Most often the formations, even when anisotropic, are relatively isotropic in the horizontal plane. This particular version of anisotropy is often called vertical transverse isotropy (VTI). Accurate imaging requires that such anisotropy be accounted for during the migration process. When sufficiently precise, such imaging enables reservoirs to be delineated from surrounding formations, and further indicates the presence of formation boundaries, laminations, and fractures, which information is desired by the reservoir engineers as they formulate a production strategy that maximizes the reservoir's economic value.
It should be understood, however, that the specific embodiments given in the drawings and detailed description do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed in the scope of the appended claims.